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Old 16th July 2006, 08:46 PM   #11
Fuling is offline Fuling  Sweden
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Quote:
Perhaps there is something I'm not noticing.
I think I know what:
Note that both stages in this amp are inverting, the output stage is not a follower but a common source stage with a P channel mosfet.
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Old 16th July 2006, 08:50 PM   #12
Jeb-D. is offline Jeb-D.  United States
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Your right regarding the inductor. I just realized impedance of most speakers is higher at lower frequencies due to resonance. So that inductor should work. The inductor will be hogging current at low frequencies but the speaker will require less current at low frequencies due, to the higher impedance.

Thanks for pointing out the output stage is a P-Channel. Your right, I'm so used to tubes, I forget solid state has devices that conduct the other way. Sorry, I'm a dumbass.
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Old 16th July 2006, 09:21 PM   #13
Fuling is offline Fuling  Sweden
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The reactance of an 112mH choke is 14 ohms at 20Hz, a bit more would be nice of course but size, weight and DC resistance (in particular, since it affects the operation point at DC) are issues that cannot be neglected.

Tubes...donīt mention them!
I was up all night (literally) trying to make an ECC86 capable of driving a simple Mosfet buffer. No matter how I tried with CCSīs, buffers and ĩ-followers they never made it to 20kHz.
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Old 17th July 2006, 06:42 PM   #14
Fuling is offline Fuling  Sweden
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I wonder...

Iīve read so much good things about cascode stages lately, would a cascoded input stage be an improvement in this circuit?
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Old 17th July 2006, 08:24 PM   #15
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Quote:
Originally posted by Fuling

Iīve read so much good things about cascode stages lately, would a cascoded input stage be an improvement in this circuit?
Only guesswork would know it without building.
I applied the cascoding with few projects to my satisfaction.
Not for reinforcement of bass, but for less distorsion. . .
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Old 17th July 2006, 08:42 PM   #16
Fuling is offline Fuling  Sweden
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Less distortion is always (?) attractive.
I guess the only way to find out is to try, just as you say.
Will do that some day, right now Iīm busy assembling a couple of TDA2005-based kits for an experimental biamp project.
For some reason these kits donīt feel as solid as my choke-amp prototype
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Old 17th July 2006, 08:51 PM   #17
Ryssen is offline Ryssen  Sweden
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Quote:
Only guesswork would know it without building.
Thatīs so true,but itīs better to ask sommeone who has already done the mistakes...
I was just thinking of a question, hmm..what is the gain diffrences with a cascode and a regular voltamp with a CCS?someone who has already found that out?
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Old 22nd July 2006, 08:25 PM   #18
Fuling is offline Fuling  Sweden
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I just ordered some part for a 4 channel version of this amp, and this time Iīm going to use BF245C instead of BF245B.
This should allow (with some changes in resistor values) higher current through the input stage and thus better capability to deliver whatever the Mosfets needs at high frequencies.
Speaking of Mosfets, I ordered some IRF9530 instead of 9640.
The lower power and voltage rating means lower input capacitance, yes?

Iīve also been shopping some alu sheets at the hardware store for the chassis, itīs time to get past the prototype stage.
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Old 22nd July 2006, 10:00 PM   #19
Fuling is offline Fuling  Sweden
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Default refining things a bit

Since Iīm intending to build four channels of this amp for my biamped speakers (FE167Eīs with stereo subs) Iīd like to adjust the bass and HF amps for their different purposes: The subwoofers requiers more power, higher damping factor and more than twice the gain compared to the HF amps.

Max output power is limited to about 10-15W by the chokes, they canīt handle more than 2A DC. 10W is though enough, and I prefer 10W SE class A over 200W from some cheap commercial AB amp...
For the FE167Eīs I only need a few watts, 4W would be perfectly adequate.

Regarding the damping factor I believe it can be altered by changing the open loop gain, right?
Higher open loop gain = more feedback = higher DF, as I understand it.
Replacing the Jfetīs drain resistor (500R) with a CCS should increase the O-L gain quite a bit I figure.
Same thing if I decrease the Mosfets source resistanse (Rs), since Rs inserts degenerative (current) feedback in the output stage.
Closed loop gain can be adjusted by trimming the 33R resistor, if we choose 11R the gain should be about x10 which would be just right for my needs.

For the HF amps I want quite the opposite: low DF and about x4-5 gain. A plain resistor as a load for the Jfet and a quite high source resistance for the mosfet. 100/33R in the feedback loop should set the gain to x4.

The one with the cascoded input stage is just an idea to improve the HF performance, though I still donīt know how/if cascoding affects the gain of the input stage.
I believe that with the same bottom Jfet and load resistor the gain should be the same as for a non-cascoded stage, but Iīm not sure.
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Old 24th July 2006, 09:41 PM   #20
Fuling is offline Fuling  Sweden
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A picture showing the prototype and some metal that hopefully will turn into a nice chassis one day:
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